The present invention pertains to surface machining of semiconductor wafers, and in particular to chemical/mechanical polishing (CMP) of silicon and other types of semiconductor wafers.
In the commercial production of semiconductor wafers, a semiconductor wafer undergoes successive operations in which relatively thin layers of conductive, semiconductive and dielectric materials are formed on one of the wafer""s major surfaces by metalization, sputtering, ion implantation and other conventional techniques. Although the thickness of such layers is measured in terms of microns or micro inches, the exposed surfaces must be polished flat, in preparation for successive layering operations.
A variety of equipment is commercially available for planarizing and otherwise preparing wafer surfaces using a variety of techniques, including chemical/mechanical polishing (CMP) processes. Typically, the layered surface (device side) of the wafer is placed face down on a polish pad carried on a rotating table. A chemically active media, which may also have abrasive particles, is introduced onto the polish table from a dispenser. This media migrates between the wafer and the polish pad.
A carrier and compressible backing pad apply a downforce to the back side of the wafer, pressing the device side of the wafer against the polishing pad surface. The polishing pad is supported in place by a polishing tub. An inner sidewall of this tub provides a fence around the circumference of the pad. A guard arm extends laterally over the moving polish pad surface and functions to condition this surface as the wafer is polished.
The carrier applying downforce to the wafer is rotatably driven about a vertical axis so as to rotate the wafer with respect to the moving polish pad surface, thereby increasing the relative motion between the wafer and the polish pad. The carrier and hence the wafer is also reciprocated back and forth along an arc, usually intersecting a radial line originating at the center of the polish pad. An optical measuring device, such as that manufactured by NOVASCAN, is coupled to the polishing system. This device optically measures the thickness of the wafer and sends a signal to the polishing system when the proper thickness is obtained.
Wafers are polished in batches. At the beginning of a polishing process a technician is required to program a number of parameters into the system""s computer. Such parameters include defining (i) the polishing interval, (ii) the rotatably-driven speed of the carrier, (iii) the number of wafers in the batch, and (iv) other related specifications.
In order to maintain the wafer underneath the carrier despite sideways or lateral dislodging forces, the carrier exerts a suction force to the backside of the wafer. This force is often insufficient to retain the wafer, thereby causing it to become dislodged. It has been observed in commercial wafer polishing operations that, despite precautions to the contrary, a dislodged wafer is thrown with destructive force against the fence of the polishing tub. In certain cases, a thrown wafer was observed to break after coming to rest on the polish pad, when it then crashes against either the laterally extending carrier or guard arm.
A sensor mounted on the carrier detects a thrown wafer condition. An audible alarm is then immediately sounded. A technician typically shuts down the system completely to remove the broken wafer. At this time, the optical measuring device is reset and the system reinitialized. This can consume time and cost.
The present disclosure is directed to an improved wafer polishing apparatus and method of operating such apparatus. A method of facilitating quick restart of a polishing process on a previously thrown non-broken wafer, is disclosed that carries out processing a wafer, identifying a thrown wafer condition, transmitting a signal to the main controller indicative indicating the thrown wafer condition, and causing, an element to be raised to prevent contact with a thrown wafer.